Aberrant activation of the PI3K pathway leading to increases in AKT activity is seen in many prostate cancer patients refractory to androgen blockade therapy. A number of small-molecule inhibitors of PI3K, including BKM120 (Novartis), have entered clinical trials. As with PI3K-AKT activation, highly expressed Pim kinase has been shown to drive the growth of prostate cancer. Recent studies have demonstrated that Pim kinase plays a critical role in promoting resistance of breast cancer to PI3K inhibition. Similarly, when the Pim-1 protein kinase is overexpressed, prostate cancer cell lines become resistant to PI3K inhibitor. In both tissue culture and animal models, the combination of a Pim inhibitor, PIM447, and BKM120 acted synergistically in blocking the survival and growth of prostate cancer cells. We therefore investigated the biochemical mechanism by which Pim-1 mediates cell resistance to the PI3K inhibitor. Previously, we have demonstrated that knockout of Pim kinases led to accumulation of reactive oxygen species (ROS) accompanied by a lower level of expression of ROS scavengers. To explore whether Pim-1 overexpression reverses this phenotype in prostate epithelial cancer, using lentiviral transduction, we expressed Pim-1 in mouse prostate epithelial cancer cells derived from spontaneous prostate adenocarcinomas that arose in a mouse model in which Tp53 and Rb1 are deleted in prostate epithelium. Pim-1 overexpression promoted anchorage-independent growth and formation of mouse prostate epithelial tumors in vivo. Gene expression arrays demonstrated that Pim-1 induced an Nrf2 antioxidant response element gene signature, including increases in Heme oxygenase-1 (Hmox1), NAD(P)H quinone oxidoreductase 1 (Nqo1), and glutathione S-transferase kappa1 (Gstk1) genes. Similarly, overexpression of Pim-1 in human prostate cancer LNCaP cells results in increased expression of Nrf2 and Hmox1 protein. As expected, depletion of Nrf2 with shRNA prevented Pim-1-mediated increases in Hmox1 expression, indicating that Nrf2 activation is required for the Pim-1 induction of ROS scavengers. In line with this result, in Pim-1 overexpressing cell lines knockdown of either Hmox1 or Nrf2 restored sensitivity to BKM120 treatment. A vital role of ROS scavengers in mediating the resistance to BKM120 was further confirmed by the treatment of cells and xenografted tumors with buthionine sulfoximine (inhibitor of glutathione synthesis), which resensitized the Pim-1 overexpressing cells to BKM120 via ROS induction. Our data further demonstrated that Pim-1 overexpression decreases the ubiquitination of Nrf2 protein, thus preventing its degradation, and increasing its half-life. Conversely, inhibition of Pim kinase promoted Nrf2 ubiquitination and proteasomal degradation. Taken together, our results show that the increased expression of Pim kinases in prostate cancer blocks therapeutic efficacy of PI3K inhibitor by inducing Nrf2 and antioxidant scavengers that inhibit the induction of ROS.
Citation Format: Jin H. Song, Libia A. Leuvano, Neha Singh, Andrew S. Kraft. NRF2 antioxidant signaling mediates the ability of PIM protein kinases to induce resistance of prostate cancer to PI3K inhibitor therapy [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr B097.